After the resonant tank is designed and the primary side MOSFET is selected, the ZVS operation of the converter needs to be double checked. ZVS can only be achieved when there is enough current left in the resonant inductor at the gate turn off edge to discharge the switch node. UC256604 implements adaptive dead-time based on the slewing of the switch node. The slew detection circuit has a detection range of 0.1V/ns to 200V/ns.

To check the ZVS operation, a series of time domain simulations are conducted, and the resonant current at the gate turn off edges are captured. An example plot is shown below:

Figure 8-1 Adaptive Dead-Time

The figure above assumes the maximum switching frequency occurs at 5% load, and system starts to burst at 5% load.

From this plot, the minimum resonant current left in the tank is I_min = 0.8A in the interested operation range. In order to calculate the slew rate, the primary side switch node parasitic capacitance must be known. This value can be estimated from the MOSFET datasheet. In this case, C_switchnode = 400pF. The minimum slew rate is given by:

Equation 43.

This is larger than 0.1V/ns minimum detectable slew rate.